Binding bioassays such as immunoassays, DNA hybridization assays, and receptor-based assays are widely used as diagnostic tests for a wide range of target molecules. Binding assays exploit the ability of certain molecules, herein referred to as “binding molecules,” to specifically bind target molecules. Binding molecules such as antibodies, strands of polynucleic acids (DNA or RNA) and molecular receptors, are capable of selectively “binding” to such potential target molecules as polynucleic acids, enzymes and other proteins, polymers, metal ions, and low molecular weight organic species such as toxins, illicit drugs, and explosives.
In a solid-phase binding assay, binding molecules are attached to a solid substrate, a procedure generally performed by the manufacturer of the assay. These binding molecules are referred to as “capture” molecules. When the user initiates the assay by exposing the solid substrate to a liquid sample, capture molecules immobilize target and/or label molecules on the surface via recognition events.
Through the use of labeled binding molecules, such recognition events can be made to generate a measurable signal and thereby indicate the presence or absence of a target molecule. Various types of binding assays have been devised that use radioactive, fluorescent, chemiluminescent, magnetic and/or enzymatic labels. Depending on the type of assay being performed, labeled binding molecules either bind to immobilized target molecules, i.e., a “sandwich” assay, or compete with target molecules to bind to capture molecules, i.e., a “competitive” assay. After removal of excess label from the sample, the amount of bound label may be measured.
In addition to the typical binding assays described above, new technologies have created additional ways to identify the target molecules in bioassays. In one specific example, certain nanoscale magnetic beads have successfully been utilized to detect the presence of various target molecules in bioassays. In this application, the magnetic beads, typically a magnetite Fe2O3 bead, are activated or “tagged” with a biochemical coating that selectively bonds with the biomolecule of interest in a given solution. Once tagged in this fashion, the magnetic beads are placed into the solution where they diffuse to a magnetoresistive sensor and attach themselves to a molecule-specific biochemical coating. The presence, or non-presence, of the tagged beads at the magnetoresistive sensor can be measured based upon the magnetic properties of the beads.
The magnetoresistive sensor can detect changes in the Giant Magnetoresistance (GMR) that is directly related to the influence of the fringing magnetic fields emanating from the beads that are attached to the biochemical coating in relatively close proximity to the magnetoresistive sensor. These magnetite beads are typically about 1–2 micrometers in diameter. These relatively large beads are necessary, given the relatively low sensitivity of the GMR sensor. It should be noted that larger beads can be used to enhance the signal, but larger beads will also tend to produce non-specific binding. Non-specific binding reduces both sensitivity and selectivity and typically increases as the beads increase in size. In addition, several target molecules may be required to adequately bind a larger bead and this may also reduce sensitivity since the presence of a single bead does not indicate the presence of a single target molecule unless it can be bound to only one target molecule.
While the use of magnetic beads to detect target molecules in a solution has been successfully demonstrated, certain practical implementation details have suggested probable limitations on the current technology. For example, the GMR sensor sensitivity is somewhat limited and, accordingly, may limit the ability of the sensor to detect relatively low levels of target molecules in a given solution. The GMR sensor sensitivity is a function of GMR magnitude (maximum resistance change) and the magnetic field response (slope of the magnetic resistance associated with the magnetic field). Presently known GMR sensors have demonstrated a sensitivity of approximately 1-microvolt signal for a single bead. Accordingly, it is possible that relatively small amounts of a target molecule in a solution remain undetectable using the presently known GMR sensors.
In view of the foregoing, it should be appreciated that it would be desirable to enhance the accuracy and sensitivity of bioassays performed using magnetic labels. In addition, it would be desirable to provide new methods and techniques for fabricating sensors without requiring the addition of new and costly procedures. Furthermore, additional desirable features will become apparent to those skilled in the art from the drawings, foregoing background of the invention, following detailed description of the drawings, appended claims, and abstract of the invention.